When I clean my ARs I cringe at the thought of getting sand in the bolt and carrier. There is some close-tolerance metal moving very fast in there.

I am not a huge fan of the gas-piston ARs. The groups that come out of those guns are 1 1/2 to 2". That is what a decent AK 74 (or 47) will do for a LOT cheaper price.

The deal with me is it takes me a while to buy a gun because when I buy one I never sell them. Bought the old lady a sweet Taurus CIA 850 .38 special Stainless... it was used and sitting in the case for 250, clean and no scratches, I had to have it.

List #1 are cartridges that will work with a standard AR-15 or M16 lower. There are several other cartridges, as well, including several WSSM chamberings and wildcats based on the .223 case. List #2 requires a longer, .308-size upper and lower receiver. As you can see, I added in a few additional cartridges for you that are currently popular in this platform.

One consideration is that inexpensive 'plinking' ammo is not readily available in many of these other cartridges. Several of them are also 'barrel-burners,' and will erode the throat of a barrel very quickly - especially in full auto shooting. The hotter a barrel gets, the faster it erodes. A full auto rifle in one of these cartridges would be a very expensive proposition.

I'm sure that several people will be along shortly to add their favorite cartridge that I didn't mention. There are several, and I didn't try to list them all.

I'm no expert on NFA technicalities, but I believe that you have to list all of the different chamberings of your NFA weapons on the appropriate ATF form, and update it when they change.